1. Field of the Invention
The present invention relates to a one port type SAW resonator and a surface acoustic wave (SAW) filter that are included in, for example, a band-pass filter or other filter for use in portable telephones and other communication devices.
2. Description of the Related Art
SAW filters are widely used as band-pass filters in portable telephones and other communication devices. In recent communication systems, such as portable telephones, the transmitting frequency band and the receiving frequency band are close to each other. Therefore, attenuation characteristics in the vicinity of the ends of the passband, that is, sharpness in the attenuation characteristics, are increasingly required.
To meet the above-described requirements, a composite SAW filter is disclosed in Japanese Unexamined Patent Application Publication No. 7-131290. In this composite SAW filter, a first SAW resonator is connected in parallel to one of an input terminal filter and an output terminal of a SAW filter, and a second SAW resonator is series-connected thereto.
For the above-mentioned prior art, a high impedance in the vicinity of an antiresonant frequency in the first SAW resonator series-connected is used to provide the sharpness in cutoff characteristics of the high-band side of the passband of the SAW filter. It is also described that a low impedance in the vicinity of a resonant frequency in the second SAW resonator connected in parallel is used to provide the sharpness in cutoff characteristics of the low-band side of the passband of the SAW filter.
In the above-described method according to the prior art, the antiresonant frequency in the first SAW resonator must be arranged to be closer to the passband on the high-band side of the passband. Also, the resonant frequency in the second SAW resonator must be arranged to be closer to the passband on the low-band side of the passband.
However, when the antiresonant frequency in the first SAW resonator is arranged to be closer to the passband, a high impedance in the vicinity of the antiresonant frequency affects the high-band side of the passband. This increases insertion losses on the high-band side of the passband. Similarly, when the resonant frequency in the second SAW resonator is arranged to be closer to the passband, the low impedance in the vicinity of the resonant frequency influences the low-band side of the passband. This increases the insertion loss on the low-band side of the passband.
That is, the above-described method according to the prior art causes a problem in that the insertion loss in the passband is increased when the amount of attenuation in the vicinity is very close to the passband.
FIG. 25 is a graph showing frequency-amplitude characteristics, which is used to explain the aforementioned inverse effects that occur when the SAW resonator is connected in parallel to the SAW filter.
In FIG. 25, broken lines indicate a frequency-amplitude characteristic of a simple substance of the SAW filter, and solid lines indicate a characteristic provided when a SAW resonator of which an impedance-frequency characteristic indicated by a broken line in FIG. 15 is connected in parallel to the above-described SAW filter.
A graph on a magnified scale shows the characteristics magnified on a scale on the right of the vertical axis. The figure showing a frequency-amplitude characteristic, which is referred to below, is similarly presented.
As is apparent in FIG. 25, when the SAW resonator is connected in parallel, amounts of attenuations increase in the vicinity of the high-band side of the passband, particularly, in frequency zones where amounts of attenuations increase from 10 dB.
However, when the resonant frequency of the SAW resonator is arranged to be close to the passband, the low-band side of the passband is influenced according to the influence of the low impedance in the vicinity of the resonant frequency. This indicates that, as shown by the solid lines, the insertion loss increases. As a result, when the sharpness on the low-band side of the passband is judged using the frequency pitch in positions where amounts of attenuations are 3 dB and 20 dB as a criteria, the frequency interval in the simple substance of the SAW filter is 3.3 MHz while it is only 3.6 MHz when the SAW resonator is connected in parallel. Thus, no improvement in the sharpness has been achieved.
FIG. 26 is a graph showing frequency-amplitude characteristics, which is used to explain the aforementioned inverse effects that occur when the SAW resonator is series-connected to the SAW filter. In FIG. 26, broken lines indicate a frequency-amplitude characteristic of the simple substance of the SAW filter, and solid lines indicate a characteristic provided when a SAW resonator of which an impedance-frequency characteristic indicated by a broken line in FIG. 18 is series-connected to the SAW filter.
As is apparent in FIG. 26, when the SAW resonator is series-connected to the SAW filter, amounts of attenuations increase in the vicinity of the low-band side of the passband, particularly, in the vicinity of 913 MHz, which corresponds to the antiresonant frequency of the SAW resonator. However, similar to the above, the high-band side of the passband is affected by the influence of the high impedance in the vicinity of the antiresonant frequency. When the sharpness in the frequency-amplitude characteristic on the high-band side of the passband is judged using the frequency pitch at positions where amounts of attenuations are 3 dB and 8 dB as a criteria, the frequency interval in the simple substance of the SAW filter is 2.2 MHz while it is 3.4 MHz when the SAW resonator is series-connected. Thus, no improvement in the sharpness has been achieved.
To prevent the decrease in the passband, that is, the adverse effects in the insertion loss, in the case of the parallel connection, the vicinity of the antiresonant frequency in the SAW resonator is simply arranged so as to agree with the passband. In the case of the series connection, the vicinity of the resonant frequency in the SAW resonator is simply arranged so as to agree with the passband. As a result of the actual connection, as described above, however, the resonant frequency in the case of the series connection is farther from the vicinity of the passband, thereby disabling large amounts of attenuations in the vicinity very close to the passband. That is, according to the conventional method in which the SAW resonator is connected to the SAW filter, large amounts of attenuations in an area that is very close to the passband and preferable insertion losses in the passband are incompatible.
To overcome the problems described above, preferred embodiments of the present invention provide a SAW resonator and a SAW filter including the SAW resonator, the SAW resonator being arranged to control the frequency interval between the resonant frequency and the antiresonant frequency and adapted to define a ladder circuit and various other types of SAW filters, and furthermore, being adapted to be connected to the SAW filter in the above-mentioned composite SAW filter.
Preferred embodiments of the present invention also provide a composite SAW filter in which the SAW resonator of the present invention is series-connected to and/or connected in parallel to the SAW filter, thereby achieving sharpness in filter characteristics in the vicinity of the passband, and concurrently, achieving insertion losses in the passband.
A SAW resonator according to a preferred embodiment of the present invention includes a piezoelectric substrate and an interdigital transducer (which is abbreviated as an xe2x80x9cIDTxe2x80x9d, hereinbelow) on the piezoelectric substrate, the IDT including first and second comb-shaped electrodes having one or more electrode fingers which are interdigitated with each other, wherein, when the first comb-shaped electrode is connected to a positive potential, the second comb-shaped electrode is connected to a negative potential, and the electrode finger connected to the positive potential and the electrode finger connected to the negative potential are reversed in at least one pair of the electrode fingers in an area where electrode fingers connected to the positive potential and electrode fingers connected to the negative potential are alternately arranged in the direction of surface-wave propagation.
A SAW resonator according to another preferred embodiment of the present invention includes a piezoelectric substrate and an IDT on the piezoelectric substrate, the IDT including first and second comb-shaped electrodes having one or more electrode fingers which are interdigitated with each other, wherein the IDT is subjected to one of withdrawal weighting and electrode reversal, and also, the effective-electrode ratio in the IDT is in a range of about 10% to about 80%. The electrode reversal refers to a configuration wherein the electrode finger connected to the positive potential and the electrode finger connected to the negative electrodes according to the preferred embodiment described above are reversed, and the meaning of the electrode reversal is described below in detail.
In the SAW resonators according to the preferred embodiments of the present invention described above, the frequency interval between a resonant frequency and an antiresonant frequency is preferably in a range of about 5% to about 75% of the frequency interval between a resonant frequency and an antiresonant frequency in a regular IDT having the same number of the electrode-finger pairs.
Also, the effective-electrode ratio in the IDT is preferably in a range of about 10% to about 50%.
In a specific case of the SAW resonators according to the preferred embodiments described above, the frequency interval between the resonant frequency and the antiresonant frequency is preferably in a range of about 5% to about 30% of the frequency interval between the resonant frequency and the antiresonant frequency in the IDT having the same number of the pairs.
Also, in the SAW resonators according to the preferred embodiments described above, reflectors may be provided outside of the IDT in the direction of surface-wave propagation direction.
According to another preferred embodiment of the present invention, a composite SAW filter is provided; and in the composite SAW filter, at least one of the SAW resonators according to the preferred embodiments described above is electrically series-connected and/or connected in parallel to a SAW filter via at least one of an input-end side and an output-end side of the SAW filter.
In a specific example of this preferred embodiment, the SAW resonator is series-connected to the SAW filter, and the antiresonant frequency is a frequency in a stopband in the vicinity of the high-band side of the passband of the SAW filter.
In another specific example of the SAW filter according to this preferred embodiment, the SAW resonator is connected in parallel to the SAW filter, and the resonant frequency is available in a stopband in the vicinity of the low-band side of the passband of the SAW filter.
According to another preferred embodiment of the present invention, a SAW filter having a ladder-type circuit configuration is provided. In the SAW filter having the ladder-type circuit configuration, multiple SAW resonators are configured in series arms and shunt arms, and at least one of the SAW resonators is defined by one of the SAW resonators according to the preferred embodiments described above.
According to yet another preferred embodiment of the present invention, a SAW filter having a ladder-type circuit configuration is provided and in which multiple SAW resonators are arranged in series arms and shunt arms, thereby defining the ladder-type circuit. Also, the effective-electrode ratio in an IDT in at least one of the SAW resonators is in a range of about 10% to about 95%.
According to a SAW resonator according to a preferred embodiment of the present invention, when a first comb-shaped electrode is connected to a positive potential, a second comb-shaped electrode is connected to a negative potential, and the electrode finger connected to the positive potential and the electrode finger connected to the negative potential are reversed in at least one pair of the electrode fingers, that is, electrode reversal is performed, in an area where electrode fingers connected to the positive potential and electrode fingers connected to the negative potential are alternately arranged in the direction of surface-wave propagation. Therefore, the effective-electrode ratio is reduced to be lower than that of a regular-type IDT, thereby allowing the frequency interval between a resonant frequency and an antiresonant frequency. Accordingly, the frequency interval between the resonant frequency and the antiresonant frequency can be adjusted by controlling the amount of the electrode reversal. Therefore, by connecting the SAW resonator as one of a series trap and a shunt trap to a SAW filter, sharpness in filter characteristics in the vicinity of the passband is greatly increased with almost no influence being exerted on the passband.
In addition, in the SAW resonator of a preferred embodiment of the present invention, since the electrode reversal, not withdrawal, is used to reduce the frequency interval between the resonant frequency and the antiresonant frequency, the area of the IDT portion is greatly reduced.
In a SAW resonator according to another preferred embodiment of the present invention, one of the withdrawal weighting and the electrode reversal is performed, and the effective-electrode ratio in the IDT is within a range of about 10% to about 80%. Therefore, similarly to the preferred embodiment described in the preceding paragraph, the frequency interval between the resonant frequency and the antiresonant frequency can be reduced to be smaller than if the regular-type IDT is used. Therefore, using the SAW resonator of this preferred embodiment as one of the series trap and the shunt trap for the SAW filter allows the sharpness in filter characteristics in the vicinity of the passband to be greatly increased with almost no influence being exerted on the passband.
In preferred embodiments of the present invention, when the frequency interval between the resonant frequency and the antiresonant frequency is controlled to be in a range of about 5% to about 75% of the frequency interval between the resonant frequency and the antiresonant frequency in the regular-type IDT, using the preferred embodiment of the present invention as one of the series trap and the shunt trap of the SAW filter allows the sharpness in the filter characteristics in the vicinity of the passband to be increased even more efficiently, and also, allows increase in the amount of attenuation in bands that are somewhat remote from the passband to be realized much more efficiently.
In the SAW resonator of preferred embodiments of the present invention, when the effective-electrode ratio in the IDT is within a range of about 10% to about 50%, connecting the SAW resonator as one of the series trap and the shunt trap to the SAW filter allows the sharpness in the filter characteristics in the vicinity of the passband to be increased even more efficiently, and also, allows desirable insertion losses in the passband to be realized even more efficiently.
In preferred embodiments of the present invention, when the frequency interval between the resonant frequency and the antiresonant frequency is controlled to be within a range of about 5% to about 30% of the frequency interval between the resonant frequency and the antiresonant frequency in the IDT, the aforementioned effective-electrode ratio can be controlled to be in a range of about 10% to about 50%. Therefore, as described above, using preferred embodiments of the present invention as one of the series trap and the shunt trap of the SAW filter allows the sharpness in the filter characteristics in the vicinity of the passband to be increased even more efficiently, and also, allows insertion losses in the passband to be sufficiently great.
In a composite SAW filter according to yet another preferred embodiment of the present invention, since at least one of the SAW resonators according to preferred embodiments described above is electrically series-connected and/or connected in parallel to at least one of an input-end side and an output-end side of the SAW filter, the sharpness in the vicinity of the passband is greatly increased, and also, insertion losses in the passband are effectively reduced.
In the SAW filter of this preferred embodiments of the present invention, when the SAW resonator is series-connected to the SAW filter and when the antiresonant frequency is arranged to be a frequency in a stopband in the vicinity of the high-band side of the passband of the SAW filter, the aforementioned SAW resonator functions as a series stopband, thereby allowing amounts of attenuations in the vicinity of the passband in the high-band side of the passband to be greatly increased, and also, allows desirable insertion losses in the passband to be realized.
In the composite SAW filter of this preferred embodiment of the present invention, when the SAW resonator is connected in parallel to the SAW filter and when the resonant frequency is arranged to be a frequency in a stopband in the vicinity of the low-band side of the passband of the SAW filter, it functions as a shunt stopband, thereby allowing amounts of attenuations in the vicinity of the passband in the low-band side of the passband to be greatly increased, and also, allows reduction in insertion losses in the passband to be implemented.
In a SAW filter according to another preferred embodiment of the present invention, multiple SAW resonators are arranged in series arms and shunt arms, and at least one of the SAW resonators is configured according to the above-described preferred embodiments of present invention. Therefore, the sharpness in filter characteristics in the vicinity of the passband is greatly increased.
Also, in a SAW filter according to another preferred embodiment of the present invention, multiple SAW resonators are arranged in series arms and shunt arms, thereby configuring a ladder-type circuit, and the effective-electrode ratio in an IDT in at least one of the SAW resonators is controlled to be in a range of about 10% to about 95%. Therefore, similarly to the preferred embodiments described above, the sharpness in filter characteristics in the vicinity of the passband are greatly increased.
For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.